Canonical correlation analysis (CCA) identifies linear relationships between multicomponent predictors and multicomponent predictands, e.g. pattern-to-pattern relationships in space and/or time. Like simpler forms of linear regression, CCA minimizes squared errors in hindcasting the predictands from the predictors. During the last decade, CCA has been used increasingly in the atmospheric sciences (e.g. Barnett and Preisendorfer 1987; Graham et al. 1987a, 1987b; Barnston and Ropelewski 1992; Barnston 1994, Barnston and He 1996). Here, CCA is used to predict 3-month precipitation anomalies in the Pacific Islands out to a year in advance, as described in He and Barnston (1996). Because rainfall in the tropical and subtropical Pacific is strongly related to ENSO (Ropelewski and Halpert 1987, 1996), it is reasonable to expect usable skill in seasonal Pacific rainfall forecasts, and thus worthwhile to establish a real-time prediction system for the benefit of commercial interests in the Pacific Islands. The experimental forecasts shown in this quarterly Bulletin are provided on a monthly basis on the Internet at address: http://nic.fb4.noaa.gov:80/products/predictions/experimental/pacific.

The predictor fields used for the forecasts include quasi-global sea surface temperature (SST), Northern Hemisphere 700 mb geopotential height, and the predictand precipitation itself (at 33 island stations) at an earlier time. CCA sensitivity experiments indicate that the SST field is the most valuable predictor field, with 700 mb heights and prior precipitation somewhat helpful. Further details about the skills, the underlying relationships, and the predictors are provided in He and Barnston (1996). The set of predictors is configured as four consecutive 3-month periods prior to the time of the forecast, followed by a variable lead time, and then a single 3-month predictand period. The predictand includes 3-month total rainfall at 33 Pacific Island stations within 25^oN-30^oS, including 4 Hawaiian stations (see Fig. 1 of any of the 1997 issues of this Bulletin). The lead time is defined as the time between the end of the final (fourth) predictor period (i.e., the time of the forecast) and the beginning of the 3-month predictand period. The set of stations predicted is expected to increase to at least 50 sometime during 1998, and to include stations close to the equator near the date line and eastward (i.e., in Kiribati). The rainfall data and climatology for the larger set of stations has been described in a recently published atlas (He et al. 1998).

The expected skill of the forecasts was estimated using a 1-year-out cross-validation (see He and Barnston 1996). The skill estimates indicate that at 1 month lead time the highest correlation skill across the Pacific Islands occurs in Jan-Feb-Mar at 0.44 (0.29) averaged over all stations north (south) of the equator, and the lowest occurs from September through December at about 0.15 (0.30) for stations north (south) of the equator. At four months lead, the skill is only slightly lower except for the Jan-Feb-Mar average skill north of the equator which drops significantly to 0.26. When skill is averaged only for periods with significant ENSO signals, the average skill is considerably higher than the above. This would be expected in view of the increase in the signal-to-noise ratio in the rainfall when ENSO-neutral periods are removed.

Figure 1 shows forecasts of the standardized precipitation anomaly (X100) for 33 Pacific Island stations using data through May 1997. The top panel shows the forecast for Jul-Aug-Sep 1998 (1 month lead), and the middle and bottom panels for Oct-Nov-Dec 1998 and Jan-Feb-Mar 1999 (4 and 7 months lead), respectively. The expected skill for these forecasts, based on cross-validation, is shown by the size of the numerals (as opposed to their value, which is the forecast itself): Small numerals indicate low skill (correlation below 0.3), medium sized numerals usable but modest skill (correlation between 0.3 and 0.45), and large numerals moderate or better skill (0.45 and higher). A weak tendency toward a continuation of dryness at off-equator locations, and enhanced rainfall at the stations closest to the equator near and east of the date line, is being forecast for Jul-Aug-Sep. This pattern is associated with the strong El Nino conditions that developed during mid-1997 and has now dissipated (and even reversed along the equator in the 110-150 ^oW region). However, residual warm SST just north and south of the immediate equator is expected to allow weak El Nino-like effects to continue for another 1 to 3 months at locations other than those along the immediate equator. If a La Nina develops during the next few months, the climate along the immediate equator from the date line eastward would be expected to become anomalously dry without any lag time, while the off-equatorial locations (e.g. Micronesia, New Caledonia) could continue to experience mildly dry conditions due to the residual warm waters from earlier in 1998. For northern fall and winter 1998-99 the forecast skill is generally low at this time. However, there are clear hints that by Oct-Nov-Dec the rainfall pattern could switch to that of a La Nina, with dryness developing along the equator east of the Date Line and mildly above normal rainfall in many of the regions that have had severe rainfall deficits over the last year due to the El Nino. Given that much of the forecast skill in this part of the world comes from ENSO episodes, confidence in the forecasts is proportional to the certainty of the future ENSO state. While it is not certain that a significant La Nina is now developing, there is at least some confidence that this may be the case in view of the large amount of below-normal sea temperatures beneath the surface in the central and eastern Pacific (see Fig. 3 in the article by Barnston et al. in this issue), and the fact that the easterly trades which returned to normal in early May will continue to cause upwelling along the equator - upwelling that had been "shut off" during the El Nino.

More detailed forecasts for 9 U.S.-affiliated and 18 non-U.S.-affiliated Pacific Island stations are shown in Fig. 2, in the form of long-lead rainfall forecasts from 1 to 13 seasons lead (solid bars) along with their expected skill (lines). The horizontal axis reflects the lead time, whose corresponding actual target period for this forecast is indicated in the legend along the top of the figure (e.g. 1=Jul-Aug-Sep 1998). The same ordinate scale is used for both the forecasts and the skill (standardized anomaly and temporal correlation coefficient, respectively). Sometimes skill may increase as the lead is increased because a more forecastable target season has been reached. The forecasts and their skill differ not only due to their differing ENSO-responsiveness caused by general location differences the Pacific basin, but also due to differences in orientation with respect to the local orography (if any).

The dry conditions that have dominated the climate at many of the U.S.- affiliated stations over the last year are expected to give way to more normal rainfall, and perhaps even enhanced rainfall by northern summer and/or fall. Some dryness could continue, however, in milder form, in parts of Micronesia during summer. South of the equator at the non-U.S.-affiliated islands, dry conditions in Jul-Aug-Sep 1998 are expected for stations farthest away from the equator and west of 170^oE (e.g. Koumac, Noumea, Luganville). Funafuti is predicted to continue to have heavier than normal rainfall through at least September. While rainfall conditions associated with La Niña could set in by August or September in locations close to the equator, it is not clear at this time whether the currently rapidly cooling SST in the east-central tropical Pacific will stabilize as a mild or moderate La Niña or continue to cool to the extent of a strong La Niña. Most statistical models, and some of the dynamical models, are indicating a substantial degree of cooling by October.

The CCA modes (not shown; He and Barnston 1996) clearly show ENSO as the dominant influence on tropical Pacific climate, especially during the months of Nov-Dec-Jan-Feb-Mar-Apr-May (and even earlier than Nov. Along the immediate equator near and somewhat east of the dateline). The current forecasts show a marked transition from strong El Niño conditions to normal, followed by a possible onset of La Niña conditions. As discussed above, locations along the immediate equator to the east of the date line where waters are currently becoming cold (e.g. eastern Kiribati) will react immediately (with dryness) to a developing La Niña, while locations off the equator may continue to have residual effects from the El Niño and experience a substantial (several months) lag before La Niña-like rainfall conditions set in.

References:

Barnett, T.P. and R. Preisendorfer, 1987: Origins and levels of monthly and seasonal forecast skill for United States surface air temperatures determined by canonical correlation analysis. Mon. Wea. Rev., 115, 1825-1850.

Barnston, A.G., 1994: Linear statistical short-term climate predictive skill in the Northern Hemisphere. J. Climate, 7, 1513-1564.

Barnston, A.G. and C.F. Ropelewski, 1992: Prediction of ENSO episodes using canonical correlation analysis. J. Climate, 5,1316-1345.

Barnston, A.G. and Y. He, 1996: Skill of CCA forecasts of 3-month mean surface climate in Hawaii and Alaska. J. Climate, 9, 2579-2605.

Graham, N.E., J. Michaelsen and T. Barnett, 1987a: An investigation of the El Niño-Southern Oscillation cycle with statistical models. 1.Predictor field characteristics. J. Geophys. Res., 92, 14251-14270.

Graham, N.E., J. Machaelsen and T. Barnett, 1987b: An investigation of the El Niño-Southern Oscillation cycle with statistical models. 2. Model results. J. Geophys. Res., 92, 14271-14289.

He, Y. and A.G. Barnston, 1996: Long-lead forecasts of seasonal precipitation in the tropical Pacific islands Using CCA. J. Climate, 9, 2020-2035.

He, Y. A.G. Barnston and A.C. Hilton, 1998: NCEP/Climate Prediction Center Atlas No. 5: A precipitation climatology for stations in the tropical Pacific basin; effects of ENSO. U.S. Dept. of Commerce, NOAA, 280pp.

Ropelewski, C.F. and M.S. Halpert, 1987: Global and regional scale precipitation patterns associated with the El Niño/Southern Oscillation. Mon. Wea. Rev., 115, 1606-1626.

Ropelewski, C.F., and M.S. Halpert, 1996: Quantifying Southern Oscillation-precipitation relationships. J. Climate, 9, 1043-1059.

Figure Captions: 

Fig. 1. CCA-derived precipitation standardized anomaly forecast (X100) for 33 Pacific Islands stations for (1) Jul-Aug-Sep 1998, Oct-Nov-Dec 1998 and Jan-Feb-Mar 1999. Latest data for these forecasts is May 1998. The cross-validated skill expected for the forecasts is indicated by the size of the numerals (not their value, which shows the forecast itself). Small numeral size indicates correlation skill of less than 0.30, considered unusable; medium size is used for 0.30<skill<0.45 which is modest but usable; large size indicates skill>0.45, considered a relatively more reliable forecast.

Fig. 2. Time series of CCA-based long-lead precipitation anomaly forecasts, and their expected skills, out to one year into the future for 9 U.S.-affiliated Pacific Island stations (first page) and 18 non-U.S.-affiliated stations (second page and third page). The bars indicate the forecast values (as standardized anomalies) and the lines indicate the associated skills (as temporal correlation coefficients). Both the forecasts and the skill use the same ordinate scale. The target season is indicated on the abscissa, ranging from 1 (Jul-Aug-Sep 1998) through 13 (Jul-Aug-Sep 1999); see the legend at top.